Split the 1200+ line mime_responds_test into 3 more focused and manageable test files.
Fail informatively in ActionController::Metal::Responder#api_behavior when no appropriate renderer is available.

Currently if a user calls #respond_with(csvable), but has not csv renderer available, Responder will just run through the default render behavior
twice, raising ActionView::MissingTemplate both times.

This changes #api_behavior to check in advance whether there is a renderer
available, and raise ActionController::MissingRenderer if not.